// Get the customer emails
    $orderEmailsCollection = Mage::getModel('sales/order')->getCollection()->addAttributeToSelect('customer_email');

    // Call iterator walk method with collection query string and callback method as parameters
    // Has to be used to handle massive collection instead of foreach
    Mage::getSingleton('core/resource_iterator')->walk($orderEmailsCollection->getSelect(), array(array($this, 'fillArray')));

    $collection = Mage::getModel('newsletter/subscriber')->getCollection()
        ->addFieldToSelect(array('subscriber_id','subscriber_email','subscriber_status'))
        ->addFieldToFilter('subscriber_email',array('nin'=>$this->filterOrdersEmails));

$this->filterOrdersEmails[] = $args['row']['customer_email'];